Field of the Invention
The present invention relates to an image pickup apparatus which improves quality of a captured image.
Description of the Related Art
When a bright object is captured by using an image pickup apparatus including s solid-state image sensor, blown-out highlights (luminance saturation) may occur in a captured image. In a region (luminance-saturated region or saturated pixel region) where the luminance saturation occurs, luminance information outside a luminance range of the image sensor (i.e., information relating to a pixel value greater than a saturated pixel value) is lost.
Previously, a method of synthesizing (combining) a plurality of images captured with different exposure values to generate a High Dynamic Range (HDR) image where the luminance range of the image sensor is widened to be captured is known. The exposure value is an index indicating a degree of exposure, which is determined depending on an aperture value of an image pickup system and an exposure time. The plurality of images used for the generation of the HDR image can be obtained for example by HDR bracket photographing where continuous photographing is performed with a plurality of different exposure values.
In this case, in order to obtain a high-quality image where the luminance saturation is sufficiently reduced, an image that is captured with a sufficiently-large exposure value and that is appropriate for a bright object in an angle of view is required. Accordingly, when the HDR photographing (HDR bracket photographing) is to be performed, it is necessary to repeat reading pixel values while previously changing the exposure value and acquire a sufficiently-large exposure value by using a photometry device. In other words, when the HDR photographing is to be performed, it is necessary to prepare to acquire an appropriate exposure value based on the plurality of images or an output value of the photometry device.
Japanese Patent Laid-open NO. 2008-104009 discloses a method of acquiring an exposure value based on a previously-prepared table by using a cumulative pixel value histogram of a previously-captured single image to perform the preparation described above. The literature “CVPRW 2012, JUNE 2012, GRADIENT DOMAIN COLOR RESTORATION OF CLIPPED HIGHLIGHTS” discloses a method of estimating a luminance value based on a single image.
As described above, when the HDR bracket photographing is to be performed, it is necessary to prepare to acquire the appropriate exposure value based on the plurality of images or the output value of the photometry device, and thus it requires time. In this regard, in the method disclosed in Japanese Patent Laid-open No. 2008-104009, the preparation can be performed based on a single image. However, in the method disclosed in Japanese Patent Laid-open No. 2008-104009, the exposure value is acquired based on the cumulative pixel value histogram of the single image, and accordingly it is difficult to acquire a sufficiently-large exposure value that is appropriate for a bright object in an image.